Gonadal steroids regulate reproductive behavior in many vertebrate species, including humans. Much is known of how steroids mediate changes in anatomical attributes of brain regions controlling reproductive behavior. The complexity of these brain regions, however, has made it difficult to identify physiological mechanisms by which steroids activate and modify behavior. Investigating the actions of steroids on the biophysical properties of steroid-sensitive neurons is important to understand how steroid actions on neurons modify behavior. The neural circuit controlling the electric organ discharge of weakly electric fish is an ideal system in which to study steroid actions on neurons. These neural circuits are steroid-sensitive and relatively simple. It is therefore possible to analyze the biophysical properties of the neurons that comprise these circuits. The goals of this proposal are; (1) to map steroid receptors in the CNS of weakly electric fish and thus identify putative sites of steroid action; (2) to determine if steroids modify the electric organ discharge by changing the endogenous firing frequencies of neurons that control the behavior; and (3) to determine if steroids change the firing frequencies of these neurons by altering the density or kinetics of ionic conductances of these neurons. By addressing these questions, we will better understand how steroid actions on neurons modify behavior and how ionic conductances contribute to neuronal oscillations underlying rhythmic behaviors.